1. Field of the Invention
This invention concerns the continuous casting between two rolls of thin metallic products, especially made of steel.
According to this known technique, the manufactured product, for example a thin strip of steel several millimetres thick, is obtained by pouring molten metal into a casting space defined between two rolls with parallel axes, cooled and rotated in opposite directions. The metal, when it comes into contact with the cold walls of the rolls, called sleeves, solidifies and the skins of the solidified metal, rotated by the rolls, join at the neck between the rolls to form the said strip which is extracted downwards.
The use of the casting process between rolls is submitted to various constraints relevant both to the cast product and the use of the casting installation.
In particular, the section of the cast strip must correspond, in shape and dimensions, to the required section, the real section of the strip being directly dependent on the space, called the gap, between the rolls at the neck.
2. Description of the Related Art
For this, a regulation process for continuous casting between rolls, described in patent application FR-A-2728817 is known, where the rolls separating force (RSF) is measured and the relative position of the said rolls modified to suit. This process enables the relative position of the rolls to be modified; they are moved apart if the force is too high or moved together if the force is too low especially in order to avoid breakouts of liquid metal or even rupture of the cast strip, and also prevent damage to the rolls in case of over-solidification of the cast metal.
Also, it is known that an out-of-round of the rolls cannot be totally avoided, on the one hand, for mechanical reasons and, on the other hand, due to the thermal deformations to which the sleeve is subjected when it first comes into contact with the molten metal when casting is started and also later during the rotation of the rolls. A compensation process for this out-of-round, which will be called hereafter "normal out-of-round" (or again "mechanical out-of-round" even though it is partly of thermal origin), is already known; this process consists of automatically acting on the position of the bearings of at least one of the rolls depending on the angular position of these rolls in order to maintain the gap as constant as possible. As it is practically impossible to directly measure the gap, it has already been proposed to use as a parameter representative of the out-of-round, a signal delivered by rolls separating force measurement means, the out-of-round compensation system then being combined with a regulation system such as described in above mentioned document FR-A-2728817.
However, the use of these processes does not enable real-time detection of certain defects liable to disturb the casting process, to lead to its shutdown or to durably damage the rolls.
Defect detection methods, visual or others, are already known enabling the detection of defects related to the casting process, to the thermal/dynamic characteristics of the molten metal, or again those known as "shiny strips". The latter type of defect corresponds to a local reduction in the surface roughness of the rolls which leads to variations in the cooling of the strip which can be detected by temperature measurements made on the cast strip. However, the observation of these defects can only be done after the event, on the already formed strip, and therefore quite sometime after they have appeared. Now, these defects can damage the surface finish of the rolls and this especially when they are perceived at a late stage, in which case the damage may be irreparable.
Certain defects can be detected a priori from direct observation of the signal representing the rolls separating force. However, variations in this signal represent both variations in the force due to the normal out-of-round and variations due to other parameters or events which may occur during casting. Direct observation of the force signal therefore does not allow to determine the part that each of these causes plays in the variations of the signal to be determined.